net-http 0.7.0 incompatible with currencycloud-ruby [causing crash]

[emilylittle]

The latest version of the net-http gem doesn't set content type by default ruby/net-http#207 (comment), so when we upgraded net-http from 0.6.0 to 0.7.0 it broke the currencycloud gem, which presumably doesn't set content type.

[matid]

@emilylittle This was fixed in httparty 0.24: jnunemaker/httparty@59c0ac5

I opened a PR (#96) since 0.24 also includes a high severity security fix.