fix `rake test` to work.

rake test doesn't work.
No errors, but doesn't execute any test/**/*_test.rb tests.
add .ruby-version and .ruby-gemset for development.
remove spec.add_development_dependency "minitest" to avoid following warnings:

    MiniTest::Unit::TestCase is now Minitest::Test.

minitest 5 rename MiniTest::Unit::TestCase to Minitest::Test.
But use minitest 4 that ruby bunled for this gem.

If minitest 5 is already installed, uninstall by following commands:

    gem list | grep minitest
    gem uninstall minitest
Merge pull request #2 from meltedice/fix-rake-test

fix `rake test` to work.
change Rgraphum::Graph#load easy to add load strategy.

To add new strategy, add following method:

    Rgraphum::Graph#build_graph_from_#{format}(graph, options)

ex)

    Rgraphum::Graph.load(format: :my_builder, options)

This invoke following method.

    Rgraphum::Graph#build_graph_from_my_builder(graph, options={})
Merge pull request #3 from meltedice/graph-builder-strategy

change Rgraphum::Graph#load easy to add load strategy.
bump version to 0.0.2-alpha
add graph builder strategy method name pattern: "build_#{options[:format]}"

before:
    "build_graph_from_#{options[:format]}".to_sym
after:
    "build_graph_from_#{options[:format]}".to_sym
    "build_#{options[:format]}".to_sym
Merge pull request #4 from meltedice/graph-builder-strategy-new-method-name-pattern

add new graph builder strategy method name pattern for rgraphum extension
bump version to v0.0.3.alpha
fix Rgraphum::Graph.build() to return build_* method return value.