Update dependency pgtk to v0.18.0

Author: renovate[bot]

Gemfile.lock

@@ -54,7 +54,7 @@ GEM
     differ (0.1.2)
     docile (1.4.1)
     drb (2.2.3)
-    elapsed (0.0.1)
+    elapsed (0.1.0)
       loog (> 0)
       tago (> 0)
     erb (5.0.1)
@@ -142,7 +142,7 @@ GEM
       ast (~> 2.4.1)
       racc
     pg (1.5.9)
-    pgtk (0.17.3)
+    pgtk (0.18.0)
       backtrace (~> 0.4)
       concurrent-ruby (~> 1.3)
       joined (~> 0.3)