I recommend to go straight to 2.1.5+ without intermediate steps. Otherwhise you burden yourself with unnecessary work of encoding problems.
Issues you may encounter:
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Set the ruby version within your .ruby-versionfile to2.1.5
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Remove gem ruby-debugand use e.g.byebug
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Remove gem oniguruma
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Remove gem fastercsv
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Replace gem mysqlwithmysql2
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Update gem capistrano 2.12.0to~>2.12when bound forRuby 1.8.7and remove obsolete explicite Gemfile entries fornet-scpandnet-sshif present.
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Update gem andand
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Update gem parallel_tests(0.8.x to 1.6.x)
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Downdgrade database_cleaner from 1.4.xto1.2.0
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Use new lambdasyntax: changelambda { |param| code }to->(param) { code }
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Unbind gem 'i18n', '~> 0.6.0'and update
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fix yml erros: - 
%{count}errors by surrounding with", e.g.: replaceone: %{count} Objektwithone: "%{count} Objekte"
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date: order: [ :day, :month, :year ]todate: order: - :year - :month - :day
 
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- 
fix incompatible character encodings: UTF-8 and ASCII-8BITerror by addingEncoding.default_external = Encoding::UTF_8 Encoding.default_internal = Encoding::UTF_8to environment.rbbeforeAppname::Application.initialize!
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use ' 'for nbsp helper instead of0xC2.chr + 0xA0.chr
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TextHelper#truncatechanged signature from(string, int)to(string, length: int)
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You may update gem sanitizefrom2.xto4+
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In regexes \wdoes not match umlauts and other special characters. If it is supposed to you can use[[:alnum:]]instead.