Bump `binding_of_caller` gem version to 0.8.0

What does this MR do?

We recently bumped ruby version to 2.5.3. Since then, spring has not been working properly as the problem is reported in https://stackoverflow.com/questions/48123534/spring-stopping-rails-console-from-running.

Luckily, this problem has already been resolved with binding_of_caller version 0.8.0 (https://github.com/BetterErrors/better_errors/issues/411#issuecomment-381852337). I actually tested locally and fixed the problem.

With binding_of_caller gem v0.7.2

shinya@shinya-MS-7A34:~/workspace/thin-gdk$ ./thin-ctl rails exec bin/rails console
Running via Spring preloader in process 1856
/home/shinya/workspace/thin-gdk/service/rails/src/spec/factories/projects.rb:4: warning: already initialized constant PAGES_ACCESS_LEVEL_SCHEMA_VERSION
/home/shinya/workspace/thin-gdk/service/rails/src/spec/factories/projects.rb:4: warning: previous definition of PAGES_ACCESS_LEVEL_SCHEMA_VERSION was here
Loading development environment (Rails 4.2.10)
History file not loaded: string contains null byte
before_session hook failed: NoMethodError: undefined method `reject!' for nil:NilClass
Traceback (most recent call last):
/home/shinya/workspace/thin-gdk/service/rails/cache/gems/spring-2.0.2/lib/spring/application.rb:171:in `fork': undefined method `reject!' for nil:NilClass (NoMethodError)

With binding_of_caller gem v0.8.0

shinya@shinya-MS-7A34:~/workspace/thin-gdk$ ./thin-ctl rails exec bin/rails console
Running via Spring preloader in process 10600
/home/shinya/workspace/thin-gdk/service/rails/src/spec/factories/projects.rb:4: warning: already initialized constant PAGES_ACCESS_LEVEL_SCHEMA_VERSION
/home/shinya/workspace/thin-gdk/service/rails/src/spec/factories/projects.rb:4: warning: previous definition of PAGES_ACCESS_LEVEL_SCHEMA_VERSION was here
Loading development environment (Rails 4.2.10)
History file not loaded: string contains null byte
[1] pry(main)>

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