upgrade java compiler source to 1.8 and add a type annotations for Re…

[henrywoo]

To justify the use of response.close() which people often use, we need to specify NotNull for Response, so I added a type annotation. It failed with an error:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:compile (default-compile) on project tchannel-core: Compilation failure
[ERROR] /home/henry/share/company/uber/tchannel-java/tchannel-core/src/main/java/com/uber/tchannel/api/SubChannel.java:[245,27] type annotations are not supported in -source 1.7
[ERROR]   (use -source 8 or higher to enable type annotations)
[ERROR] 

This conflicts with the comment in pom.xml:

<!-- store method parameter names in the bytecode (java 8+) -->

It seems we forgot to specify java 8. So I modified it. That is how this diff comes.

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[henrywoo]

The Travis CI build failed because it is still using Java 1.7. Could anybody in charge of this project please upgrade it to 1.8? Thanks.