remove unecessary development dependency; bundler

[MatzFan]

Fixes #16

[MatzFan]

Couldn't run tests due to typos:

$ bundle exec rspec
An error occurred while loading spec_helper.
Failure/Error: Dir.mkdir(CERT_DIR) unless File.exists?(CERT_DIR)

NoMethodError:
  undefined method 'exists?' for class File
# ./lib/evil-proxy/quickcert.rb:61:in '<class:QuickCert>'
# ./lib/evil-proxy/quickcert.rb:54:in '<top (required)>'
# ./lib/evil-proxy/mitmproxy.rb:3:in '<top (required)>'
# ./lib/evil-proxy.rb:3:in '<top (required)>'
# ./spec/spec_helper.rb:4:in '<top (required)>'
No examples found.

File.exists? should be File.exist?.