PARQUET-1198: Bump java source and target to java8

Author: Gabor Szadovszky <gabor.szadovszky@cloudera.com>

Closes apache#452 from gszadovszky/PARQUET-1198 and squashes the following commits:

5b7ee44 [Gabor Szadovszky] PARQUET-1198: Bump java source and target to java8

Author: Gabor Szadovszky

pom.xml

@@ -67,8 +67,8 @@
   </developers>
 
   <properties>
-    <targetJavaVersion>1.7</targetJavaVersion>
-    <maven.compiler.source>1.7</maven.compiler.source>
+    <targetJavaVersion>1.8</targetJavaVersion>
+    <maven.compiler.source>1.8</maven.compiler.source>
     <maven.compiler.target>${targetJavaVersion}</maven.compiler.target>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <github.global.server>github</github.global.server>