you need to prove that $H^1(S^1)$ is torsion-free (it's clear that it's finitely generated). Note that $\pi_n(S^1) = 0$ for $n\geq 2$ by the covering $\Bbb R \to S^1$ and LES of homotopy groups for fibrations, so that $S^1 = K(\pi_1(S^1),1)=K(H_1(S^1),1))$ (by Hurewicz + $\pi_1$ of top. groups is abelian). If there was any torsion in $\pi_1(S^1)=H_1(S^1)$, then the cohomological dimension of $\pi_1(S^1)$ would be infinite,
contradicting the fact that it is finite since $S^1$ is a finite-dimensional manifold