[tox] Add missing dependency on webtest for devtools tests

[tox] Add missing dependency on webtest for wsgi tests

[tox] Remove set of PATH environment variable The : concatenation does not work actually so the resulting PATH was just wrong (it did not contain the previous PATH). Anyways, the PATH environment variable is passed by default by tox so it's probably better to let the caller set it (thus avoiding to hard-code the specific postgres version and path). http://tox.readthedocs.org/en/latest/example/basic.html#passing-down-environment-variables