Add missing semicolon in externs

[jkuszmaul]

No description provided.

[coveralls]

Coverage increased (+0.02%) to 90.199% when pulling 28560a9 on jkuszmaul:master into da2a028 on danvk:master.

[danvk]

Thanks. Are you using dygraphs with Closure Compiler?

[jkuszmaul]

I am not compiling dygraph myself; I'm taking the released dygraph.min.js you provide and using the externs file with closure to get the declarations.

I should probably make a pull request for them as well, but I also had to:

• Add an @externs tag in the file comment for the externs file.
• Changed the HTMLDivElement to just an Element, because I was too lazy to find the proper way to get an HTMLDivElement object out of the DOM (I'm not a javascript person normally--I just needed to create a small plotting webpage for a project).