Make the spec in pip_install compatible with pip specifications.

Currently, commands like `conda install cherrypy==3.2.2` or `conda install cherrypy=3.2.2` do not work because the spec computed using arg2spec() does not contain any equal signs but if conda decides to use pip as a failback installer, the spec needs to add back the equal signs since pip needs them.
Merge pull request #350 from desilinguist/patch-1

Make the spec in from_pypi.pip_install() compatible with pip specifications.