add lambda support for default country code

[kimyu92]

• connected to issue default_country_code based on relation #174
• allow custom attribute from current model/ attribute from association
• run travis against latest patch of rubies and add 2.4 & 2.5 versions into yml file
• minor bump on phony gem

[coveralls]

Coverage increased (+0.002%) to 99.813% when pulling 2c94d17 on kimyu92:master into e131c50 on joost:master.

[joost]

Awesome. Is it useful to add some documentation. I will merge it already.