Fix naming conflict with StandardError

[tachyons]

Ruby standard Error have a builtin method from ruby 2.1 onwards which
returns the exception object which caused the current error
See: http://ruby-doc.org/core-2.3.1/Exception.html#method-i-cause

Using the same variable for storing the error cause string can side effects
as in #59

Fixes #59

[tachyons]

Build failure is unrelated, addressed here #63

[tachyons]

 with recrusive, waits and return when the key is updated (FAILED - 1)

Failures:

  1) Etcd watch with recrusive, waits and return when the key is updated

     Failure/Error: res = http.request(req)

     

     Net::ReadTimeout:

       Net::ReadTimeout

     # ./lib/etcd/client.rb:111:in `api_execute'

     # ./lib/etcd/keys.rb:81:in `watch'

     # ./spec/etcd/watch_spec.rb:41:in `block (3 levels) in <top (required)>'

     # ------------------

     # --- Caused by: ---

     # IO::EAGAINWaitReadable:

     #   Resource temporarily unavailable - read would block

     #   ./lib/etcd/client.rb:111:in `api_execute'

Build failure seems due to another issue which is not deterministic
@ranjib

[ranjib]

Thank you :-)