Move from agignore to gitignore

Per @andyw8 in #588:
https://github.com/ggreer/the_silver_searcher/wiki/Advanced-Usage

`agignore` is now deprecated as the place to declare ignored files for
search. We'll put these files into `gitignore` rather than `ignore` so
that we have consistently ignored files between ag, git, etc.

Recommended by @croaky:
bcfa239b31

Close #588

agignore

@@ -1,7 +0,0 @@
-.git/
-tags
-/bower_components/
-/log
-/node_modules/
-/tmp
-/vendor

gitignore

@@ -1,13 +1,18 @@
-.DS_Store
+!tags/
+*.pyc
 *.sw[nop]
+.DS_Store
 .bundle
 .byebug_history
 .env
+.git/
+/bower_components/
+/log
+/node_modules/
+/tmp
+/vendor
 db/*.sqlite3
 log/*.log
 rerun.txt
 tags
-!tags/
 tmp/**/*
-!tmp/cache/.keep
-*.pyc