Update minimum JDK requirements.

The minimum was bumped to 8 in:
  https://rbcommons.com/s/twitter/r/4127/

Testing Done:
Verified the minimum with:
```
$ ./pants clean-all publish.jar \
  --no-prompt \
  --no-dryrun \
  --force \
  --local=/tmp/repo src/java/::
$ mkdir /tmp/junit
$ unzip -qd /tmp/junit/ \
  /tmp/repo/org/pantsbuild/junit-runner/1.0.14-SNAPSHOT/junit-runner-1.0.14-SNAPSHOT.jar
$ find /tmp/junit -name "*.class" | \
  xargs file | \
  cut -d: -f2 | \
  sed -E "s|^\s+||" | \
  sort -u
compiled Java class data, version 52.0 (Java 1.8)
```

IOW, the next publish of any org.pantsbuild support jar will contain
java 8 classfiles as opposed to the current crop of jars which all
still contain java 6 classfiles.

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/161763813

Bugs closed: 3892

Reviewed at https://rbcommons.com/s/twitter/r/4253/

README.md

@@ -24,5 +24,5 @@ At a minimum, pants requires the following to run properly:
 * Linux or Mac OS X
 * Python 2.7.x (the latest stable version of 2.7 is recommended)
 * A C compiler, system headers, Python headers (to compile native Python modules)
-* OpenJDK 7 or greater, Oracle JDK 6 or greater
+* OpenJDK or Oracle JDK version 8 or greater
 * Internet access (so that pants can fully bootstrap itself)