Remove redundant parts from Travis config file.
Remove redundant parts from Travis config file.

--use-mirrors has been deprecated, and pip install .  likely isn't needed

pierrelb/RMG-Py,nickvandewiele/RMG-Py,KEHANG/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,faribas/RMG-Py,faribas/RMG-Py,enochd/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,comocheng/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,KEHANG/RMG-Py