Change formatter plugin to @diffplug's Spotless plugin

[jbduncan]

Mandatory disclaimer: I am a contributor to Spotless, so I'm naturally biased towards it, but despite this please allow me to explain why I think we should adopt this plugin over the plugin we currently use. :)

1. Error messages are (in my opinion) easier to understand.
   For example, we currently have messages like

Found 1 non-complying files, failing build (validateOnly is true)

which aren't terribly informative - they don't explain which files no longer comply and why, they don't explain how to fix the problem, and the messages easily get lost in the noise of other Maven logging messages.
But with Spotless, we'd have messages like

The following files had format violations:
src\main\java\edu\uci\ics\jung\graph\DelegateCTree.java
@@ -203,8 +203,7 @@
··@Override
··public·boolean·removeNode(N·node)·{
····checkNotNull(node,·"node");
-····if·(!nodes()
-········.contains(node))·{
+····if·(!nodes().contains(node))·{
······return·false;
····}
····for·(N·nodeToRemove·:·Graphs.reachableNodes(delegate,·node))·{
Run 'mvn spotless:apply' to fix these violations.

which AFAICT fix the problems above rather nicely.

2. We can easily adopt formatters other than google-java-format in the future, such as a formatter which checks that files have license headers that follow a certain template. (This seems useful or even important for us, since currently the license headers in JUNG's source files are a bit inconsistent.)

3. If Spotless's Maven plugin ever exposes a "padded cell" option in the future (currently only Spotless's Gradle plugin exposes such an option), and if we ever discover that google-java-format or another formatter has bugs which prevent it from producing consistent results (idempotency bugs), then we can use "padded cell" in the short-term to ensure that that formatter continues producing consistent results until the respective bug is fixed.

[jbduncan]

@jrtom Hmm, I'm having trouble getting Travis CI to pass now. It seems there's a problem on the Spotless side (to do with unexpected CRLF line endings) which has left me stumped. So when I next have free time, I'll probably elect to close this PR and create a new one, to see if starting things afresh fixes things.

[jrtom]

I'm happy to review this (or a subsequent CL) whenever you have time to sort out what's going on.

[jbduncan]

Whoops, I obviously forgot about this PR! I'll put it at the top of my mental open-source-things-to-tackle list. :)

[jrtom]

No worries, no rush, I was just reminded of it by your comment on the Spotless issue. :)

[jbduncan]

I'm about to push a new PR to see if that fixes things, so I'm closing this PR.