Fix so that raven-sentry compiles on JDK-6.

Currently the top-level POM specifies only JDK 6 and raven-java wasn't compiling successfully against it. The error was:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.3.2:testCompile (default-testCompile) on project raven: Compilation failure
[ERROR] /Users/buckett/Documents/raven-java/raven/src/test/java/net/kencochrane/raven/connection/HttpConnectionTest.java:[122,8] cannot find symbol
[ERROR] symbol  : method assertThat(java.util.List<java.util.logging.LogRecord>,org.hamcrest.Matcher<java.lang.Iterable<? super java.lang.Object>>)
[ERROR] location: class net.kencochrane.raven.connection.HttpConnectionTest