Update README.textile
So now it suggests running `before_install_linux.sh` script using `bash` instead of `sh`,
as that script is not supported by `sh`.
@@ -187,7 +187,7 @@ h4. 3) install ImageMagick and additional environment stuff
<pre>
cd /vagrant/rmagick
export IMAGEMAGICK_VERSION=6.8.9-10
-sh ./before_install_linux.sh
+bash ./before_install_linux.sh
</pre>
This will take just a few minutes to build ImageMagick
