Set build requirements, minimum Maven version

Urge the use of <dependencyManagement> by disallowing duplicate
dependency declarations. See #262.

Making final releases with SNAPSHOT dependencies in use is a really bad
idea that can happen by accidental oversight. Enforcer helps ensure it
doesn't.

For build reproducibility it's good to specify what Maven versions are
supported for the project's build to work. 3.0.5 is the minimum required
version by all our plugins currently, according to:

    $ mvn versions:display-plugin-updates

ingestion/pom.xml

@@ -184,12 +184,10 @@
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java</artifactId>
-      <version>${protobufVersion}</version>
     </dependency>
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java-util</artifactId>
-      <version>${protobufVersion}</version>
     </dependency>
 
     <dependency>
@@ -202,7 +200,6 @@
     <dependency>
       <groupId>org.apache.kafka</groupId>
       <artifactId>kafka-clients</artifactId>
-      <version>2.3.0</version>
     </dependency>
 
     <dependency>
@@ -257,8 +254,6 @@
     <dependency>
       <groupId>com.github.kstyrc</groupId>
       <artifactId>embedded-redis</artifactId>
-      <version>0.6</version>
-      <scope>test</scope>
     </dependency>
 
     <!-- To run actual Kafka for ingestion integration test -->
@@ -269,7 +264,6 @@
       <scope>test</scope>
     </dependency>
 
-
     <dependency>
       <groupId>com.google.guava</groupId>
       <artifactId>guava</artifactId>

pom.xml

@@ -348,6 +348,54 @@
                     </compilerArgs>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-enforcer-plugin</artifactId>
+                <version>3.0.0-M2</version>
+                <executions>
+                    <execution>
+                        <id>valid-build-environment</id>
+                        <goals>
+                            <goal>enforce</goal>
+                        </goals>
+                        <configuration>
+                            <rules>
+                                <requireMavenVersion>
+                                    <version>3.0.5</version>
+                                </requireMavenVersion>
+                                <requireJavaVersion>
+                                    <version>1.8.0</version>
+                                </requireJavaVersion>
+                                <reactorModuleConvergence />
+                            </rules>
+                        </configuration>
+                    </execution>
+                    <execution>
+                        <id>consistent-dependency-versions</id>
+                        <goals>
+                            <goal>enforce</goal>
+                        </goals>
+                        <configuration>
+                            <rules>
+                                <banDuplicatePomDependencyVersions />
+                            </rules>
+                        </configuration>
+                    </execution>
+                    <execution>
+                        <id>no-snapshot-deps-at-release</id>
+                        <goals>
+                            <goal>enforce</goal>
+                        </goals>
+                        <configuration>
+                            <rules>
+                                <requireReleaseDeps>
+                                    <onlyWhenRelease>true</onlyWhenRelease>
+                                </requireReleaseDeps>
+                            </rules>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-surefire-plugin</artifactId>