Slghtly cleaned up travis file. Changed required versions of gsplot and devtools.
Slghtly cleaned up travis file. Changed required versions of gsplot and devtools.

zmoore-usgs/repgen,USGS-R/repgen,dpattermann-usgs/repgen,lindsaycarr/repgen,mhines-usgs/repgen,USGS-R/repgen,USGS-R/repgen,lindsaycarr/repgen,mhines-usgs/repgen,zmoore-usgs/repgen,dpattermann-usgs/repgen,dpattermann-usgs/repgen,zmoore-usgs/repgen,thongsav-usgs/repgen,thongsav-usgs/repgen,lindsaycarr/repgen,mhines-usgs/repgen,thongsav-usgs/repgen