Travis CI: run PHPUnit against WordPress 3.9

Fixes #153
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Fixes #170
PHPDoc fixes

See #160
Points: avoid errors displaying top users if none

Fixes #169
Merge branch 'issue-153'
Improved update user rank function

Includes unit tests.

Fixes #171
Update rank transitioning to reflect update func.

See #171
Merge branch 'issue-171'
PHPDoc: WordPoints_Rank::get_data() can ret. false

See #160