Add BUCK filename to Python in languages.yml

BUCK is the filename for the build files of the Facebook Buck build
system. BUCK files are valid Python files.

Eg.:
  https://github.com/GerritCodeReview/gerrit/blob/master/BUCK

Also add a missing sample for Pants / Bazel BUILD files. They are also
valid Python files.

lib/linguist/languages.yml

@@ -474,7 +474,7 @@ CSS:
   color: "#563d7c"
   extensions:
   - .css
-  
+
 CSV:
   type: data
   ace_mode: text
@@ -2854,6 +2854,7 @@ Python:
   - .wsgi
   - .xpy
   filenames:
+  - BUCK
   - BUILD
   - SConscript
   - SConstruct