add summary to mrbgems without it

mrbgems/mruby-bin-strip/mrbgem.rake

@@ -1,5 +1,6 @@
 MRuby::Gem::Specification.new('mruby-bin-strip') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
+  spec.summary = 'irep dump debug section remover command'
   spec.bins = %w(mruby-strip)
 end

mrbgems/mruby-eval/mrbgem.rake

@@ -1,4 +1,5 @@
 MRuby::Gem::Specification.new('mruby-eval') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
+  spec.summary = 'standard Kernel#eval method'
 end

mrbgems/mruby-exit/mrbgem.rake

@@ -1,4 +1,5 @@
 MRuby::Gem::Specification.new('mruby-exit') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
+  spec.summary = 'Kernel#exit method'
 end

mrbgems/mruby-string-utf8/mrbgem.rake

@@ -1,4 +1,5 @@
 MRuby::Gem::Specification.new('mruby-string-utf8') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
+  spec.summary = 'UTF-8 support in String class'
 end