Remove conditional dependencies from gemspec

Conditionals in a gemspec are only executed
when building the gem.

That is, when you run gem install, you get the built gem.
If the gem was built under CRuby 1.9.3, it would not specify
jruby-openssl or tlsmail as dependencies, no matter what
platform or version of Ruby you are running gem install on.

It only really makes sense to have them in the Gemfile for use in
development.
If you make a copy of a message, then change the headers of the original, it should not affect the
headers of the copy.
Change Mail::Message#initialize_copy to rely on Mail::Header#initialize_copy
Treetop is no longer a dependency
Correctly specify Gemfile platforms
Only set BUNDLE_GEMFILE from Rakefile when not set

and then only when not running in Appraisal or
on TravisCI
Ensure Travis bundles gems and runs bundle exec
Continue test when ruby-debug is missing
Prevent ruby-debug from being installed on TravisCI

The 'local_development' group is a way to specify gems
you may use while developing the gem, but aren't required
to run the tests.
Remove duplicate mime-types dep from Gemfile

The gemspec already specifies a version of mime-types.
Specifying it in the Gemfile is redundant and repetitive. :)
Widen mime-types dep to include 2.x
Update TravisCI configs to current format
Run tests against various mime-types using Appraisal

Run tests against all mime-types version with

    appraisal rake

or

    rake appraisal
Configure TravisCI to run on all gemfiles

Exclude invalid ruby/mime-types combos from TravisCI
Add instructions on running appraisal or wwtd
Correct gemfile path on TravisCI

It looks like when @travisci doesn't find the
expected gemfile, it just skips bundling altogether,
but continues with no error message.

This leads to confusing failures.
Only strip the first ocurrance of the field name
Merge pull request #713 from bf4/correct_deps

Tests against all the mime-types; correct dependencies once and for all
Updating ruby versions on travis to test before gem release
Updating readme on how to run a specific mime type test