Updated rover-df

Author: ankane

Gemfile

@@ -6,6 +6,6 @@ gem "rake"
 gem "minitest"
 gem "matrix"
 gem "numo-narray-alt", platform: [:mri, :windows]
-gem "rover-df", platform: [:mri, :windows], github: "ankane/rover" # TODO update when 0.6.0 released
+gem "rover-df", platform: [:mri, :windows]
 gem "csv"
 gem "ruby_memcheck"