src/sage/libs/gap/sage.gaprc: disable color prompt

This is currently done in gap_reset_workspace(), but I think it's less
surprising to find the GAP initialization in the GAP initialization
file. Here we add it to sage.gaprc so that it can later be removed
from gap_reset_workspace().

Author: orlitzky

src/sage/libs/gap/sage.gaprc

@@ -2,6 +2,8 @@
 # contain bug fixes/workarounds and/or any Sage-specific patches necessary for
 # Sage's libgap interface.
 
+# Disable color in the prompt, it interferes with parsing.
+ColorPrompt(false);
 
 # Load the GAP packages that GAP itself tries to autoload in the
 # default configuration (see "PackagesToLoad" in lib/package.gi). The