Fix plaintext email for lines with multiple links

Resolve broken plaintext email conversion for lines with multiple links, prev introduced with greedy regex in https://github.com/premailer/premailer/commit/b6779527b5d9c2c224027e92c98bf3dca3e704ea#diff-13d400fc965c7276f2c7f6ae54c26aebR30

Fixes broken tests:
* https://github.com/premailer/premailer/blob/master/test/test_html_to_plain_text.rb#L142
* https://github.com/premailer/premailer/blob/master/test/test_html_to_plain_text.rb#L158