Update README.md with additional requirement

[rizaon]

I am trying bud on Ubuntu 18.04.1 LTS, using rvm to install ruby-1.9.3, as follow

$ rvm install ruby-1.9.3
$ rvm use --default 1.9.3
$ gem install bud

All seems fine, except the dbm package is not compiled correctly, and rebl won't start. The dbm compilation logs look like this:
https://gist.github.com/mpapis/4355976
After some searching, I am able to compile it correctly after installing libdb-dev and libgdbm-compat-dev

$ sudo apt-get install libdb-dev libgdbm-compat-dev
$ rvm install ruby-1.9.3
$ rvm use --default 1.9.3
$ gem install bud

It would be nice to mention this solution in README.md as ruby-1.9 is quite obsolete.