don't track Gemfile.lock

Gemfile.lock

@@ -1,45 +0,0 @@
-PATH
-  remote: .
-  specs:
-    rebel (0.2.0)
-
-GEM
-  remote: https://rubygems.org/
-  specs:
-    ast (2.3.0)
-    coderay (1.1.1)
-    method_source (0.8.2)
-    minitest (5.10.1)
-    parser (2.4.0.0)
-      ast (~> 2.2)
-    powerpack (0.1.1)
-    pry (0.10.4)
-      coderay (~> 1.1.0)
-      method_source (~> 0.8.1)
-      slop (~> 3.4)
-    rainbow (2.2.1)
-    rake (12.0.0)
-    rubocop (0.47.1)
-      parser (>= 2.3.3.1, < 3.0)
-      powerpack (~> 0.1)
-      rainbow (>= 1.99.1, < 3.0)
-      ruby-progressbar (~> 1.7)
-      unicode-display_width (~> 1.0, >= 1.0.1)
-    ruby-progressbar (1.8.1)
-    slop (3.6.0)
-    sqlite3 (1.3.13)
-    unicode-display_width (1.1.3)
-
-PLATFORMS
-  ruby
-
-DEPENDENCIES
-  minitest
-  pry
-  rake
-  rebel!
-  rubocop
-  sqlite3
-
-BUNDLED WITH
-   1.14.5