Upgrade stringio for ruby-head builds

Author: rwstauner

benchmarks/erubi-rails/Gemfile.lock

@@ -184,7 +184,7 @@ GEM
       sprockets (>= 3.0.0)
     sqlite3 (1.7.3)
       mini_portile2 (~> 2.8.0)
-    stringio (3.1.1)
+    stringio (3.1.7)
     strscan (3.1.0)
     thor (1.3.2)
     timeout (0.4.1)

benchmarks/lobsters/Gemfile.lock

@@ -288,7 +288,7 @@ GEM
     sqlite3 (2.5.0)
       mini_portile2 (~> 2.8.0)
     stackprof (0.2.26)
-    stringio (3.1.2)
+    stringio (3.1.7)
     svg-graph (2.2.2)
     thor (1.3.0)
     timeout (0.4.1)