Make ListenWatcher optional closes #128

By default spring is configured to watch_via [`listen`](https://github.com/guard/listen) is available spring will use that, alternatively spring can be configured to use `:listen`, `:polling`, or a`CustomClass`.

```ruby
Spring.watch_via = :polling
```
Explicitely ask for ::Rails

I was experiencing a bug where rails was not loaded, and I was getting
an error about being unable to find Spring::Application::Rails, this
will remove that ambiguity for anyone else.

Change-Id: I017a596b5b606d7bc6913f30068a85df4a4948f8
Use a Listen's API method that was here before 1.0
Update the README regarding Listen
updated: get Gemfile path from environment variable "BUNDLE_GEMFILE"
Uniquify base directories
Merge pull request #148 from cbrunsdon/rails_namespace

Explicitely ask for ::Rails
Merge pull request #151 from rymai/use-listen-stable-api

Use a Listen stable API
Merge pull request #155 from eagletmt/unique-base-directories

Uniquify base directories
bump to 0.0.9
Handle multibyte characters in environment variables

Send byte size of JSON instead of string length as IO#read uses byte
lengths. If environment contains multibyte UTF-8 characters, byte size
is more than string length and read content is cropped.
Merge pull request #135 from kylewelsby/master

Make ListenWatcher optional. Closes #128
Rework the explanation of watching in the readme
Rename watch_via option to watch_method

It reads a bit better to me.
Don't fallback to polling

If the user has specified `Spring.watch_method = :listen`, but listen is
not available, just let it throw a `LoadError`.
Listen version must be ~> 1.0

Pre-1.0 support was broken anyway, and I don't want to maintain it.
Add changelog item for the Spring.watch_method= config option
Merge pull request #152 from YanhaoYang/master

updated: get Gemfile path from environment variable "BUNDLE_GEMFILE"
Merge pull request #158 from lautis/env-multibyte-utf8

Handle multibyte characters in environment variables
Vendor OkJson to prevent clashes with bundled json lib

Fixes #142.