Update .gitignore

according to the .gitignore generated by `mix new` in Elixir v1.7

.gitignore

@@ -1,12 +1,30 @@
-/_build
-/deps
-/doc
-/test/fixtures/umbrella/_build
-/test/tmp
-/ex_doc-*.ez
-/ex_doc-*.tar
-/erl_crash.dump
-/node_modules
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where 3rd-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+ex_doc-*.tar
+
+/node_modules/
+/test/fixtures/umbrella/_build/
+/test/tmp/
 /npm-debug.log
 
 # Ignore artifacts from non-production builds