validations: support non-V8 browsers

Call `Error.captureStackTrace()` only when it is available.

Use `this.stack = (new Error).stack` when `captureStackTrace` is not
available but the `stack` property is (Firefox).
Merge pull request #152 from strongloop/fix-validation-error-stack-in-browsers

[2.0] validations: support non-V8 browsers