Update lib/linguist/languages.yml

Add DOT language: http://www.graphviz.org/content/dot-language
Update lib/linguist/languages.yml

Add .gv (GraphViz) file extension to DOT language.
Update lib/linguist/languages.yml

Change sort order
Update lib/linguist/languages.yml

Set lexer to Text only for DOT. This hopefully fixed the failure on Travis.
Merge pull request #363 from dveeden/master

Add DOT language