Update to support up to Julia 0.7.0-DEV.3661

Compatibility with Julia 0.7.0-DEV.3661 (#53)

* Fix readstring deprecation

* Fix is_os deprecations

* Fix randstring deprecation

* Fix replace without pairs deprecation

* Fix ismatch deprecation

* Fix another read deprecation

* Fix comparison with nothing

Update for Julia 0.6

Remove Appveyor 32-bit

Running into the error `CURLE_SSL_CACERT_BADFILE (77)`. I'm unsure
at this time if it is an FTPClient issue or a LibCURL issue.