[FLINK-7032] [build] Overwrite inherited compiler versions from parent pom

Default values for compiler version are 1.6 and were causing Intellij to
constantly switch language level to 1.6, which in turn was causing
compilation errors. It worked fine for compiling from console using
maven, because those values are separetly set in maven-compiler-plugin
configuration.

This closes #4213.

pom.xml

@@ -98,6 +98,11 @@ under the License.
 		<guava.version>18.0</guava.version>
 		<akka.version>2.3-custom</akka.version>
 		<java.version>1.7</java.version>
+		<!-- Overwrite default values from parent pom.
+			 Intellij is (sometimes?) using those values to choose target language level
+			 and thus is changing back to java 1.6 on each maven re-import -->
+		<maven.compiler.source>${java.version}</maven.compiler.source>
+		<maven.compiler.target>${java.version}</maven.compiler.target>
 		<scala.macros.version>2.1.0</scala.macros.version>
 		<!-- Default scala versions, may be overwritten by build profiles -->
 		<scala.version>2.11.11</scala.version>