Should support bundler version 2.x.x

[Shekharrajak]

I got an error while using bundler 2.0 :

 ➜  bundle install
Fetching gem metadata from https://rubygems.org/...........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.10)

  Current Bundler version:
    bundler (2.0.1)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?

Could not find gem 'bundler (~> 1.10)' in any of the relevant sources:
  the local ruby installation

[Shekharrajak]

Ping @v0dro, please have a look into this PR. Otherwise, travis CI build will fail.

[Shekharrajak]

We can see in Announcing page of bundler 2, mention that it has "Removed support for Ruby < 2.3". So the system which has bundler 2 installed already, will not be able to setup daru. Running bundle install will through this error :

For Ruby <= 2.2 :

Fetching: bundler-2.0.1.gem (100%)
ERROR:  Error installing bundler:
	The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with `gem install bundler -v 1.17.3`
	bundler requires Ruby version >= 2.3.0. The current ruby version is 2.1.0.
The command "gem install bundler" failed and exited with 1 during .

For Ruby >= 2.3

Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.10)
  Current Bundler version:
    bundler (2.0.1)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?

[Shekharrajak]

Following the daru-view travis file

[Shekharrajak]

I will wait for 24 hours ( time for any other comments if someone has any other suggestions). Then I will go ahead and merge this.