Fix deprecated RSpec code

I was using NMatrix recently and found some deprecation warnings when running `rake spec` with RSpec 2.99.2:

```
The semantics of `RSpec::Core::Pending#pending` are changing in
RSpec 3.  In RSpec 2.x, it caused the example to be skipped. [...]

To keep the same skip semantics, change `pending` to `skip`.

`be_false` is deprecated. Use `be_falsey` (for Ruby's conditional semantics) or `be false` (for exact `== false` equality) instead. Called from /Users/carlosagarie/Projects/nmatrix/spec/02_slice_spec.rb:130:in `block (4 levels) in <top (required)>'.

`be_true` is deprecated. Use `be_truthy` (for Ruby's conditional semantics) or `be true` (for exact `== true` equality) instead. Called from /Users/carlosagarie/Projects/nmatrix/spec/02_slice_spec.rb:131:in `block (4 levels) in <top (required)>'.
```

I did the changes specified in these warnings (keeping the current behavior) in this patch.