curl: add missing libssh2 dependency

Homebrew was failing to install libssh2 as a curl dependency, at least when reinstalling (I previously had a bottled curl without libssh2 support):

```
🐯  brew reinstall curl --with-libssh2 --with-openssl                                                                          
==> Reinstalling curl with --with-libssh2, --with-openssl
==> Downloading http://curl.haxx.se/download/curl-7.39.0.tar.bz2
######################################################################## 100.0%
==> ./configure --prefix=/usr/local/Cellar/curl/7.39.0 --with-ssl=/usr/local/opt/openssl --with-ca-bundle=/usr/local/etc/openss
checking for libssh2_channel_open_ex in -lssh2... no
checking libssh2.h usability... no
checking libssh2.h presence... no
checking for libssh2.h... no
configure: error: libSSH2 libs and/or directories were not found where specified!
```

I reordered a few deps to match the options list earlier in the formula.