fix some warnings emitted by 'ruby -w'

- File.exists? is deprecated since 2.1.0
- tasks/mruby_build_commands.rake:268: ambiguous argument

tasks/mrbgems_test.rake

@@ -8,7 +8,7 @@ MRuby.each_target do
         g.print_gem_init_header(f)
         test_preload = [g.dir, MRUBY_ROOT].map {|dir|
           File.expand_path(g.test_preload, dir)
-        }.find {|file| File.exists?(file) }
+        }.find {|file| File.exist?(file) }
 
         g.build.mrbc.run f, test_preload, "gem_test_irep_#{g.funcname}_preload"
         g.test_rbfiles.flatten.each_with_index do |rbfile, i|

tasks/mruby_build_commands.rake

@@ -106,7 +106,7 @@ module MRuby
     private
     def get_dependencies(file)
       file = file.ext('d') unless File.extname(file) == '.d'
-      if File.exists?(file)
+      if File.exist?(file)
         File.read(file).gsub("\\\n ", "").scan(/^\S+:\s+(.+)$/).flatten.map {|s| s.split(' ') }.flatten
       else
         []
@@ -265,7 +265,7 @@ module MRuby
       # if mrbc execution fail, drop the file
       unless $?.exitstatus
         File.delete(out.path)
-        exit -1
+        exit(-1)
       end
     end
   end

tasks/mruby_build_gem.rake

@@ -2,7 +2,7 @@ module MRuby
   module LoadGems
     def gembox(gemboxfile)
       gembox = File.expand_path("#{gemboxfile}.gembox", "#{MRUBY_ROOT}/mrbgems")
-      fail "Can't find gembox '#{gembox}'" unless File.exists?(gembox)
+      fail "Can't find gembox '#{gembox}'" unless File.exist?(gembox)
 
       GemBox.config = self
       GemBox.path = gembox
@@ -25,7 +25,7 @@ module MRuby
 
       gemrake = File.join(gemdir, "mrbgem.rake")
 
-      fail "Can't find #{gemrake}" unless File.exists?(gemrake)
+      fail "Can't find #{gemrake}" unless File.exist?(gemrake)
       Gem.current = nil
       load gemrake
       return nil unless Gem.current
@@ -50,7 +50,7 @@ module MRuby
         url = params[:git]
         gemdir = "#{gem_clone_dir}/#{url.match(/([-\w]+)(\.[-\w]+|)$/).to_a[1]}"
 
-        if File.exists?(gemdir)
+        if File.exist?(gemdir)
           if $pull_gems
             git.run_pull gemdir, url
           else