Fix travis for railscasts-china deploy need mysql2
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README update.
upgrade gems
Replace debugger with byebug as debugger2 author suggested.

https://github.com/cldwalker/debugger/commit/dde7db54df56505adcdf39516b9c737a17ee58b4
upgrade gems
upgrade gems
Rails upgrade to 4.1.7

http://weblog.rubyonrails.org/2014/10/30/Rails_3_2_20_4_0_11_4_1_7_and_4_2_0_beta3_have_been_released/
upgrade gems
upgrade gems
rails upgrade to 4.1.8
Merge pull request #16 from Eric-Guo/master

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