Some of Shakespeare's plays were printed individually in quarto editions during Shakespeare's lifetime, but Julius Caesar is one of the plays that was first printed after Shakespeare's death in the so-called First Folio of 1623. As a consequence, the First Folio text of Julius Caesar is the only authoritative text of the play, and any variations you see are a consequence of decisions by editors. One such decision is to what extent to modernise punctuation and spelling. For the line you cite, the First Folio had the following:
Men at sometime, are Masters of their Fates.
The "modern spelling" edition of the Internet Shakespeare uses the following spelling and punctuation:
Men at some time are masters of their fates.
(The Internet Shakespeare notes that "at some time" means "sometimes, occasionally".)
So what you see in the Pelican and Arden editions that you cite are two degrees of modernisation, unless "sometime" in the Pelican edition is a printing error.
Update: The change from "are" to "were" appears to go beyond mere modernization, as orome pointed out in a comment. Arthur Humphreys' edition of Julius Caesar for the Oxford Shakespeare (1984, not the text included in the single-volume edition of Shakespeare's plays that is also titled "The Oxford Shakespeare) has the following note:
F reads 'at sometime', which OED defines as '... in former times, formerly', its last example being of 1579. If Shakespeare wrote 'Men at sometime were ...' Cassius, very fittingly, would be lamenting Rome's fall from sturdy independence (see ll. 150-61). But the evidence is not certain enough to warrant emending F's 'are' to 'were' (as John Jowett suggests).
('F' refers to the First Folio; OED refers to the Oxford English Dictionary; John Jowett is one of the general editors of the Oxford Shakespeare - both the series of single-play editions and the single-volume edition.)
So the Pelican edition's change to 'were' brings the verb tense in agreement with an older meaning of 'sometime' and slightly changes the meaning of Cassius' words.