Drop support for Java 6.

.travis.yml

@@ -7,7 +7,6 @@ script: ./gradlew build -s
 jdk:
   - oraclejdk8
   - oraclejdk7
-  - openjdk6
 # Caching for Gradle files, prevents hitting Maven too much.
 before_cache:
   - find $HOME/.gradle/ -name '*.lock' -print -exec rm -f {} \;

build.gradle

@@ -61,8 +61,8 @@ subprojects {
     version = '6.1.1-SNAPSHOT'
     ext.internalVersion = version + ";" + gitCommitHash
 
-    sourceCompatibility = 1.6
-    targetCompatibility = 1.6
+    sourceCompatibility = 1.7
+    targetCompatibility = 1.7
 
     checkstyle.configFile = new File(rootProject.projectDir, "config/checkstyle/checkstyle.xml")