Remove findbugs from Travis and simplify the yml configuration file

[sbesson]

The findbugs:findbugs Maven phase was added to the Travis test script as a way to flag errors. However this is generated a report while the command to fail the build is findbugs:check. The latter generates many warnings across components which cannot be fixed in the main Bio-Formats source code without significant work. Instead these should be fixed and activated in each decoupled component.

This commit removes findbugs:findbugs: from the Travis test and simplifies the YML file to clarify the fact we are testing a 2x2 matrix (Ant/Maven with JDK 7/8).

To test this PR, check Travis is green and the time of oraclejdk8/maven component is reduced to match roughly the time of the openjdk8/maven component.

[mtbc]

Should we create a card for coordinating a findbugs cleanup effort with activating findbugs:check as the last step or would that be too ambitious?

[sbesson]

@mtbc given the output of the command, my impression is certainly that activating findbugs:check at the level of the entire Bio-Formats source code would be quite ambitious. Using the ongoing decoupling of the codebase into smaller repositories to fix the split code and let the findbugs check pass seems more feasible. Thus I would be inclined to start driving this cleanup at this level - see e.g. ome/ome-common-java#3 for the ome-common component.

[rleigh-codelibre]

The changes look fine and travis is green. Please merge.

Seconded on scheduling findbugs cleanup in the split components. Doing it bioformats in due course would also be desirable if possible.