Improve utility environment variable name consistency.

Author: KelSolaar

tasks.py

@@ -255,7 +255,7 @@ def docs(ctx, html=True, pdf=True):
         Task success.
     """
 
-    with ctx.prefix('export COLOUR_SCIENCE_DOCUMENTATION_BUILD=True'):
+    with ctx.prefix('export COLOUR_SCIENCE__DOCUMENTATION_BUILD=True'):
         with ctx.cd('docs'):
             if html:
                 message_box('Building "HTML" documentation...')