Due to limitations in browsers, wrap the button text inside e.g. a span element and set the styles on it. Example (available as jsfiddle too):
<style>
.buttontext {
width: 95px;
overflow: hidden;
white-space: nowrap;
display: block;
text-overflow: ellipsis;
}
</style>
<button><span class=buttontext>Very long text Very long text
Very long text</span></button>