Fix compilation with Java 6.

I'm getting the following error when building master:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.0.2:compile (default-compile) on project parquet-hadoop: Compilation failure
[ERROR] /Users/tom/workspace/parquet-mr/parquet-hadoop/src/main/java/parquet/hadoop/ParquetInputFormat.java:[122,46] incompatible types; inferred type argument(s) java.lang.Object do not conform to bounds of type variable(s) C
[ERROR] found   : <C>java.lang.Class<C>
[ERROR] required: java.lang.Class<?>

I noticed that the Travis build is using Java 7, whereas I'm using Java 6 (1.6.0_37). The changes here fix the problem for me.