Fix issue #17: raise ServerException instead of InvalidPhoneNumberException

[jalerson]

Description

• Install webmock gem in order to stub HTTP requests
• Create a Gemfile
• Add test case
• Replace expected_codes local variable by a VALID_RESPONSE_CODES constant
• Freeze ENDPOINT (https://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/MutableConstant)
• Rename MessageBird::InvalidPhoneNumberException to MessageBird::ServerException
• Replace parent class by StandardError

Fixes https://github.com/messagebird/ruby-rest-api/issues/17

[tsyber1an]

outdated branch, please update with recent master.

[jalerson]

@Funfun done 👍