Disallow degenerate case in rhombus creation

Summary:
In `areas_of_trapezoids_rhombi_and_kites`, if the random values were drawn such that `SH == -B2`,
then the rhombus would be constructed in a way such that there wasn't room for the right angle
sign, leading to a JavaScript error. On mobile, this made such exercises uncompletable.

Test Plan:
- Run `python -m SimpleHTTPServer`.
- Navigate to `areas_of_trapezoids_rhombi_and_kites`.
- Load a bunch of times; verify that things are sane and that there are no errors.

Reviewers: emily

Reviewed By: emily

Subscribers: jared, #ios

Differential Revision: https://phabricator.khanacademy.org/D25841

exercises/areas_of_trapezoids_rhombi_and_kites.html

@@ -18,7 +18,7 @@
                     <var id="B2">randRange(2, 8)</var>
                     <var id="H">randRange(1, 6)</var>
 
-                    <var id="SH">randRangeNonZero(-2, 2)</var>
+                    <var id="SH">randRangeExclude(-2, 2, [0, -B2])</var>
 
                     <var id="K">1/2 * (B1 + B2) * H</var>
                 </div>