Fixed FSSM::Pathname.segments method to work correctly.

Maybe, this problem occurs only under the cygwin environment.
The FSSM::Pathname.segments method did not work correctly. Therefore reconnection with join method failed to insert File::SEPARATOR into between each segment.

This problem makes failure for watch sub-command of compress.

before:

    FSSM::Pathname.for("/tmp/foo").segments
    # => ["//", "tmp", "foo"]
    FSSM::Pathname.for("/tmp/foo").join("bar")
    # => #<Pathname:/tmp/foo/bar>
    # OK, no problem. But a result of segments is a little bit strange.

    FSSM::Pathname.for("//host/share/foo").segments
    # => ["/", "tmp", "foo"]
    FSSM::Pathname.for("//host/share/foo").join("bar")
    # => #<Pathname:/tmp/foo/bar>
    # OK, no problem. But a result of segments is a little bit strange.

    cache = FSSM::Tree::Cache.new
    # => #<FSSM::Tree::Cache:0x00000600730888 @children={}>
    cache.set "/tmp/foo"
    # => 2014-02-13 23:20:19 +0900
    cache.files
    # => {"//tmpfoo"=>2014-02-13 23:20:19 +0900}
    # Oops!!! The path is corrupted. It's a problem!!!

after:

    FSSM::Pathname.for("/cygdrive/c/tmp/foo").segments
    # => ["/", "cygdrive", "c", "tmp", "foo"]
    FSSM::Pathname.for("/cygdrive/c/tmp/foo").join("bar")
    # => #<Pathname:/cygdrive/c/tmp/foo/bar>
    # OK, no problem. A result of segments is also reasonable.

    FSSM::Pathname.for("//host/share/foo").segments
    # => ["//host/share", "foo"]
    FSSM::Pathname.for("//host/share/foo").join("bar")
    # => #<Pathname://host/share/foo/bar>
    # OK, no problem. A result of segments is also reasonable.

    cache = FSSM::Tree::Cache.new
    # => #<FSSM::Tree::Cache:0x00000600272878 @children={}>
    cache.set "/tmp/foo"
    # => 2014-02-13 23:20:19 +0900
    cache.files
    # => {"/tmp/foo"=>2014-02-13 23:20:19 +0900}
    # OK, fixed a problem.