Include upload sample's tests in travis

The tests of [upload](https://github.com/revel/revel/tree/develop/samples/upload) sample have been added to the CI process.

Moreover, (1) the paths of test files (those ones which are uploaded) are modified. ``PostFile`` expects full file paths. Meanwhile in my last PR (#744) I have used relative ones. Fixing it now by concatenation of file paths with ``revel.BasePath``.

(2) Code of testsuites is linted.