Correct install order of MySQL deps

Hello!
The issue #24 seems actual.

If you want to install hhvm in the situation like this:
```
Required: freetype ✔, gettext ✔, mcrypt ✔, glog ✔, oniguruma ✔, re2c ✔, libelf ✔, readline ✔,
pcre ✔, gd ✔, icu4c ✔, libmemcached ✔, tbb ✔, boost ✔, imagemagick ✔, binutils ✔, ncurses ✔, curl ✔,
imap-uw ✔, gcc48 ✔, folly ✔, jemallocfb ✔, libdwarf ✔, libeventfb ✔, boostfb ✔,
mysql-connector-c++ ✘, mysql ✘
```
the installation would fail with something like that:
```
CMake Error at FindMySQL.cm:218 (MESSAGE):
  mysql_config wasn't found, -DMYSQL_CONFIG_EXECUTABLE=...
CMake Error at FindMySQL.cm:220 (MESSAGE):
  Cannot find MySQL.  Include dir: MYSQL_INCLUDE_DIR-NOTFOUND 
```
So mysql-connector-c++ fails without MySQL. We need to install MySQL first.