Have Numeric Axis re-layout on scale update if necessary.

Calls _computeWidth() and checks if there's enough room to show
everything. If not, calls _invalidateLayout().

We should switch to using a CachingTextMeasurer for it later on.

Close #903.
Only check for width changes if Axis.Numeric is vertical.
Change BBox test to try to identify why Travis is failing.
More changes to try to diagnose Travis failure.
Widen SVG to try to get Travis to pass.
Merge pull request #905 from palantir/903-axis-width

Have Numeric Axis re-layout on scale update if necessary.