Update Meson requirement to 0.50.0
Meson 0.50.0 was released on 10th March 2019. We should now be able to depend on it.
In particular, Freedesktop SDK now contains 0.51.0[1] so this shouldn't break gnome-build-meta like last time (13948fda052e).
Fixes the following warnings:
WARNING: Project specifies a minimum meson_version '>=0.47' but uses features which were added in newer versions:
* 0.49.0: {'/ with string arguments'}
* 0.50.0: {'install arg in configure_file'}