gobject-introspection: bump require gobject-introspection version

see: https://bugzilla.gnome.org/show_bug.cgi?id=709462
https://git.gnome.org/browse/gobject-introspection/commit/?id=f6770c808203d4d3eb9a30e5c4bc896f990cae34
Merge pull request #221 from cosmo0920/fix-signal-flags-gi-1_38_0

gobject-introspection: raise require gi version to run SignalFlags assertion

Patch by cosmo. Thanks!!!

Suggested by mtasaka. Thanks!!!
pango: add missing Pango >= 1.18.0 check

pango_cairo_font_map_new_for_font_type() is available since Pango
1.18.0.

[ruby-gnome2-devel-en] Adding back check for CAIRO 1.8, in rbpangocairo.c

Reported by Grant Schoep. Thanks!!!
gdk3: remove method redefined warning

In GdkCursor, display is a property. see: https://developer.gnome.org/gdk3/3.4/gdk3-Cursors.html
gdk3: remove trailing spaces
gtksourceview2: `language-ids` and `search-path` are properties

see: https://developer.gnome.org/gtksourceview/stable/GtkSourceLanguageManager.html
gtksourceview2: `scheme-ids` and `search-path` are properties.

see: https://developer.gnome.org/gtksourceview/stable/GtkSourceStyleSchemeManager.html