Quote extension to prevent yaml from incorrectly casting the values

Fixes #26

lib/linguist/languages.yml

@@ -1,4 +1,4 @@
-# Defines all Languges known to GitHub.
+# Defines all Languages known to GitHub.
 #
 # All languages have an associated lexer for syntax highlighting. It
 # defaults to name.downcase, which covers most cases. Make sure the
@@ -315,13 +315,13 @@ Go:
 Groff:
   extensions:
   - .man
-  - .1
-  - .2
-  - .3
-  - .4
-  - .5
-  - .6
-  - .7
+  - '.1'
+  - '.2'
+  - '.3'
+  - '.4'
+  - '.5'
+  - '.6'
+  - '.7'
 
 Groovy:
   major: true