more travis compatibility

Author: sameersingh

.travis.yml

@@ -1,3 +1,7 @@
 language: java
 
+before_install:
+ - sudo apt-get update -qq
+ - sudo apt-get install gnuplot
+
 install: mvn install -DskipTests=true -Dgpg.skip=true

src/test/scala/org/sameersingh/scalaplot/ExampleTest.scala

@@ -133,7 +133,7 @@ class ExampleTest {
 
     println(output(ASCII, c))
     // println(output(SVG, c))
-    output(GUI, c)
+    // output(GUI, c) // fails on X11-less nodes
     output(PDF(dir, "pdf"), c)
     output(PNG(dir, "png"), c)
   }