Fix a typo for version number in requirements-dev.txt (#759)
Fix a typo for version number in requirements-dev.txt (#759)

The current version of requirements-dev.txt includes sphinx_rtd_theme=0.4.3. 
To use pip install -r requirements-dev.txt, it requires the version number to be 
specified using ==. I added the second = in the pull request.
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python