No. The most obvious problem is that there is more than one license called "BSD." Specifically there is a 2-clause BSD license and a 3-clause BSD license. So even if we accept the programming-language statement
s.license = 'BSD'
as a valid declaration of a license, it is ambiguous at best.
In addition, the BSD license(s) require that the BSD license itself be included in the distribution, at least in distributions of the source code. For example the 3-clause BSD license says the following:
- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
The single line of a machine-readable specification statement such as
s.license = 'BSD'
does not meet this requirement, so it is not a valid way to license something under the BSD license(s). Probably you can take it as an intention that the work be licensed under a BSD license, but I do not think it is yet a valid license, because it does not itself follow the requirements of the license that it is supposedly under (namely, to include the copyright and permission statement with the source distribution). The best thing to do would be to contact the author or file a bug report and ask him to fulfill the license requirements of the BSD (3-clause) BSD license in his own work if his intention was to license it in that way.
EDIT: Technically speaking, the above could possibly still be considered a valid license. The original author is not technically bound to himself fulfill the requirements of the license, since it is his own code. I.e. the license says what you must do in order to redistribute the code, not what the author must do. However I think it would still be best to contact the author and clarify the situation. It was probably just an oversight.
See also: Is the developer of a GPL-covered program bound by the GPL? Could the developer's actions ever be a violation of the GPL? (this is for the GPL, but the same reasoning would apply to any license).