Ci against ruby27 (#253)

* CI against Ruby 2.7

* Ruby 2.7.0 Released
https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/

* e2mmap gem is no longer bundled with Ruby 2.7

```
/home/travis/.rvm/rubies/ruby-2.7.0/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require': cannot load such file -- e2mmap (LoadError)
```

https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/

```
The following libraries are no longer bundled gems. Install corresponding gems to use these features.
* CMath (cmath gem)
* Scanf (scanf gem)
* Shell (shell gem)
* Synchronizer (sync gem)
* ThreadsWait (thwait gem)
* E2MM (e2mmap gem)
```

* `lib/irb/slex.rb` has been removed from Ruby 2.7

```
/home/travis/.rvm/rubies/ruby-2.7.0/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require': cannot load such file -- irb/slex (LoadError)
```

Refer ruby/ruby@efbca15
lsegal/yard#1290 (comment)

* Allow lower irb versions

* Add benchmark gem explicitly

.travis.yml

@@ -6,6 +6,7 @@ rvm:
   - 2.4
   - 2.5
   - 2.6
+  - 2.7
   - jruby-head
 matrix:
   include:

solargraph.gemspec

@@ -30,6 +30,9 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'thor', '~> 1.0'
   s.add_runtime_dependency 'tilt', '~> 2.0'
   s.add_runtime_dependency 'yard', '~> 0.9'
+  s.add_runtime_dependency 'e2mmap'
+  s.add_runtime_dependency 'irb', '<= 1.1.0'
+  s.add_runtime_dependency 'benchmark'
 
   s.add_development_dependency 'pry', '~> 0.11.3'
   s.add_development_dependency 'rspec', '~> 3.5', '>= 3.5.0'