Fix Git URL regex so it doesn't match Windows paths

[screamerbg]

This patch fixes the regular expression that matches Git URLs so it doesn't match Windows paths. Without this patch, the path of local (unpublished) repositories on Windows, e.g. "c:/users/path/repo.git" which only exists locally, will be identified as remote repositories. This also fixes py.test on Windows #600

D:\Work\neo>py.test
============================= test session starts =============================
platform win32 -- Python 2.7.11, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: D:\Work\neo, inifile:
collected 40 items

test\add_test.py ........
test\import_test.py ....
test\ls_test.py ....
test\remove_test.py ........
test\sync_update_test.py ................

========================= 40 passed in 550.85 seconds =========================

[theotherjimmy]

Nice small diff. It looks like it does not break linux (Circle CI passed)

[theotherjimmy]

@screamerbg I'll merge this now. Could you push a new patch release of mbed-cli when you get the chance?