Set the same minimum required gtk version for cmake and autotools

3.10 is what Ubuntu 14.04 (Trusty) ships with and that's the
most recent on Travis. So we can't require a more recent version
for now.

CMakeLists.txt

@@ -192,7 +192,7 @@ ELSE (WIN32)
 ENDIF (WIN32)
 
 IF (WITH_GNUCASH)
-  GNC_PKG_CHECK_MODULES (GTK3 REQUIRED gtk+-3.0>=3.14.0)
+  GNC_PKG_CHECK_MODULES (GTK3 REQUIRED gtk+-3.0>=3.10.0)
 ENDIF (WITH_GNUCASH)
 
 GNC_PKG_CHECK_MODULES (ZLIB REQUIRED zlib)